1. Field of the Invention
The present invention generally relates to an electric power steering system of a vehicle. More particularly, this invention relates to an electric power steering system that remains stable even when subjected to forces, such as vibrations received from road surface, from a motor or engine or other source.
2. Discussion of the Related Art
A power steering system is mounted to an automobile (or any vehicle) to assist the steering by reducing the steering force required by a driver. Electric power steering systems, as compared to conventional non-electric systems, are smaller in size and have lighter weight. Also, the number of constituent parts is relatively small.
An electric power steering system generally comprises a torque sensor for outputting a signal corresponding to a value of the steering torque of the driver, an electric motor (a kind of drive means) for driving an assist apparatus which assists the steering force, and an electronic control unit (ECU) for determining current flowing through the electric motor in accordance with the output signal of the torque sensor. That is, as the driver turns the steering wheel, the torque sensor outputs a signal corresponding to a value of the steering torque generated. The ECU determines the assist current and the electric motor drives the assist apparatus based on the torque corresponding to the assist current.
However, the problem is that such a torque sensor usually detects not only the steering torque generated by the driver""s steering operation, but also vibration received from the road surface or generated by an internal combustion engine or motor or other source of vibration. If such vibration is detected by the torque sensor, then that vibration produces a noise component that is also included in the assist current. That is, the assist force is varied also in relation to the vibration. In this case, the steering becomes unstable.
Accordingly, there is a need for an electric power steering system that is not influenced by vibration and which remains stable. The present invention satisfies this need and provides related advantages as well.
It is a purpose of the present invention to provide an electric power steering system capable of keeping the steering stable even when a torque sensor detects vibration forces other than those input by the driver, such as received from road surface, from vibrations generated from a motor or engine, or from other sources of vibrations.
As used herein, the term xe2x80x9cstable,xe2x80x9d when used in reference to the electric power steering system, refers to the system only responding to the input of the driver and not to external forces, such as vibrations.
To achieve the above purpose, a first aspect of the present invention provides an electric power steering system having a torque sensor for outputting a signal corresponding to a value of a steering torque, a derivative means for outputting a signal obtained by differentiating the output signal of the torque sensor, a driving means for electrically driving an assist apparatus which assists a steering force, and an assist current determining means for determining current flowing to the drive means in accordance with the output signal of the torque sensor and the output signal of the derivative means. The electric power steering system further has gain control means for setting a gain of the derivative means to zero or reducing the gain when the output signal of the torque sensor is smaller than a predetermined value.
According to the electric power steering system of the first aspect, the signal corresponding to the steering torque is output by the torque sensor, and the signal obtained by differentiating the output signal of the torque sensor is output by the derivative means. The assist apparatus for assisting the steering force is electrically driven by the drive means, but the current flowing to the drive means is determined by the assist current determining means in accordance with the output signal of the torque sensor and the output signal of the derivative means. When the output signal of the torque sensor is smaller than the differential value, the gain of the derivative means is set to zero or reduced by the gain control means. Thus, even if the torque sensor detects vibrations received from the road surface or vibrations generated from the motor or engine, this noise component is prevented from being included in the assist current, so as to prevent the assist force from varying with the vibration. Thus, it is possible to prevent the steering from being unstable so that the vehicle drives straight.
As described above, the torque sensor detects the vibration, but if the vibrations detected by the torque sensor are high frequency, the time between the instant when the steering torque is detected and the instant when the steering force is assisted becomes relatively long, and the assist force is adversely applied in a direction opposite from a direction to which the steering torque is applied. However, even in such a case, it is possible to prevent the steering from being unstable.
A second aspect of the present invention provides an electric power steering system having a torque sensor for outputting a signal corresponding to a value of a steering torque, a derivative means for outputting a signal obtained by differentiating the output signal of the torque sensor, a driving means for electrically driving an assist apparatus which assists a steering force, and an assist current determining means for determining current flowing to the drive means in accordance with the output signal of the torque sensor and the output signal of the derivative means. The electric power steering system further has second gain control means for setting a gain of the derivative means to zero or reducing the gain when the output signal of the derivative means is smaller than a predetermined value.
According to the electric power steering system of the second aspect, the signal corresponding to the steering torque is output by the torque sensor, and the signal obtained by differentiating the output signal of the torque sensor is output by the derivative means. The assist apparatus for assisting the steering force is electrically driven by the drive means, but the current flowing to the drive means is determined by the assist current determining means in accordance with the output signal of the torque sensor and the output signal of the derivative means. When the output signal of the derivative means is smaller than the differential value, which occurs when the variations amount per unit time of the steering torque are smaller than the predetermined value, the gain of the derivative means is set to zero or reduced by the second gain control means even if a steering torque equal to or greater than the predetermined magnitude is applied. Thus, even if the torque sensor detects vibrations received from the road surface or vibrations generated from the motor, it is possible to prevent that noise component from being included in the current flowing to the drive means, so as to prevent the assist force from being varied in terms of vibrations. Thus, it is also possible to prevent the steering from being unstable when the vehicle is turning.
As described above, the torque sensor detects the vibrations, but if the vibrations detected by the torque sensor are high frequency, the delay time (the time between the instant when the steering torque is detected and the instant when the steering force is assisted) becomes relatively long, and the assist force is adversely applied in a direction opposite from a direction to which the steering torque is applied. However, even in such a case, it is possible to prevent the steering from being unstable.
According to a third aspect of the present invention, in the electric power steering system of the second aspect, the system further has gain control means for setting a gain of the derivative means to zero or reducing the gain when the output signal of the torque sensor is smaller than a predetermined value.
The electric power steering system of the third aspect functions in the same manner as that of the second aspect, and when the output signal of the torque sensor is smaller than the predetermined value, the gain of the derivative means is set to zero or reduced by the gain control means. Thus, even if the torque sensor detects vibrations received from the road surface or vibrations generated from the motor, it is possible to prevent that noise component from being included in the current flowing to the drive means, so as to prevent the assist force from being varied in terms of vibrations. Thus, it is possible to prevent the steering from being unstable when the vehicle drives straight.
According to a fourth aspect of the present invention, in the electric power steering system of any one of the first to third aspects, the system further has second assist current determining means for setting a value of current flowing to the drive means to zero amps (0A).
The electric power steering system of the fourth aspect functions in the same manner as that of the first, second or third aspects. When the output signal of the torque sensor is smaller than the predetermined value, since the value of current flowing to the drive means is determined as 0A by the second assist current determining means, the result is that the steering remains stable. This is the case when vibrations received from the road surface or vibrations generated from the motor are detected by the torque sensor even if the gain of the derivative means is set to zero by the gain control means or the second gain control means.
The invention also provides methods for producing a stable electric power steering system by inputting a steering torque into the electric power steering system by an operator, subjecting the electric power steering system to forces other than those input by the operator, outputting a signal corresponding to a value of a steering torque from a torque sensor, outputting a signal obtained by differentiating the output signal of said torque sensor from a derivative means, electrically driving an assist apparatus which assists a steering force using a driving means, determining current flowing to said drive means in accordance with the output signal of said torque sensor and the output signal of the derivative means using an assist current determining means, and setting a gain of the derivative means to zero or reducing the gain when the output signal of the torque sensor is smaller than a predetermined value using a gain control means. This electric power steering system remains stable even when subjected to various other forces, for example vibrations from the road surface, the engine or the motor, as well as from other sources. It is also contemplated herein that the electric power steering system remains stable even when the forces other than those input by the operator are nonexistent.